Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for pre-
cipitating cellulose carbamate from an aqueous alkali solu-
tion.
Cellulose carbarnate is an alkali-soluble cellulose
derivative which is a compound of cellulose and isocyanic
acid. It is possible -to produce an alkali solution of cellu-
lose carbamate which can be spun into fibre and film form
using a precipitating bath which precipita-tes -the cellulose
carbamate from the solution.
For the precipitating bath, a solution eon-tain ng
sulphuric acid has normally been used. Cellulose carbamate is
stable in acid conditions and it is therefore not deeomposed
when precipitated. When the alkali solution of carbamate is
brought into contact with sulphuric acid,-the cellulose earba-
~ate is"precipitated and at the same time as the sodium
h~dri~ide is neutralized, sodium sulphate is formed.
Thus, the eellulose carbamate spinning process pro-
duces sodium sulphate as a by-prl~duet, whieh being low in
priee affects the eeonomie ilnabb~i~y of the process. Another,
more signifieant drawbaek is that the acid neutralizes sodium
hydroxide, whieh eannot easily be returned to the process.
The sodium hydroxide accounts for a significant part of the
raw material costs of the process. It is therefore desirable
to develop a precipitation method in whieh no undesirable by-
products are produced. A precipation- process would be par-
ticularly desirable in which there would be no need to use
mineral acid and in which at least a substantial part of the
sodium hydroxide could be recovered in an eeonomieal way.
The proeess should nevertheless meet eertain require-
ments as regards the fibre or film quality. For instanee, the
initial strength of -the fibre being preeipitated should be
suffieient so that the produet that is produced can-tolerate the
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mechanical strains which it is sub;ected to in the treatment. In
fibre manufacturing, for instance, stretching the fibre in the
spinning step is essential for achieving the desired strength
properties.
- According -to the present invention there is provided a
process for precipitating cellulose carbamate from an alkali
solution, comprising bringin~ the solution into contact with a
solution containing an inorganic salt selected from carbonate,
bicarbonate, chloride, sulphate, sulphite, and bisulphite and
Lu thereby precipitating the cellulose carbamate.
In accordanc~ with the invention, it has been found
that the cellulose carbamate can be precipitated from an alkali
solution satisfactorily, as regards fibre or film production,
1~ using a precipitation solution which instead of the conventional
sulphuric acid contains an inorganic salt.
The inventive process makes it possible to precipitate
cellulose carbamate in a solution which contains no mineral acid
at all. In the process of the invention no salts are produced as
by-products from which NaOH cannot be economically regenerated.
Furthermore, a process is achieved in which the sodium hydroxide
used for dissolving carbamate can easily be recovered and reused.
2~ The use of a salt solution as precipitating bath
enables the sodium hydroxide contained in the spinning solution
to be recovered and reused. In contrast, when sulphuric acid is
used, the sodium hydroxide is converted into sodium sulphate and
an equivalent quantity has to be introduced in the process.
3~ Sodium hydroxide accounts for a significant part of the raw
material costs; therefore it is possible with the aid of the
invention to improve substantially the economy of the process.
Another essential advantage of the process of the
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invention is the feasibility of easy recycling and recovery
of the chemicals. Particularly advantageous is the use of
carbonate solution because in that case the recovery and
recycling of the sodium hydroxide used for dissolving the
carbamate can be made possible. In princlple, it is only
necessary to ensure that the sodium hydr-
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oxLde content of the precipitating bath does not become too
high. This is achieved by withdrawing from -the precipitating
bath an equivalent of sodium hydroxide and using i-t for prepar-
ing cellulose carbamate solution. In principle, no sodium carbonate
is consumed in the process, nor any other sal-ts used in the process.
In the process of the invention, the salts contained in the precip-
itating solution are inorganic salts. Almost all salts have a pre-
cipitating effect at least to some extent, although the best results
have been achieved using carbonates,,bicarbonates, chlorides and
i ~ sulphates. Also acid salts,such as ~ ,can be used efficient-
ly as precipitates. The best results, regarding both fibre prop-
erties and recovery of chemicals appear to he attainahle when car-
bonate, sodium carbonate in particular, is used for the salt.
The required salt quantity depends, for instance, on which salt is
used and on its solubility properties at the temperatures employed.
Too low salt content has the consequence that the carbamate does
not precipitate fast enough, whereby the strength properties are
impaired. When sodium carbonate is used, the appropriate salt con-
tent in the precipitating solution is 10-30~ by weight, preferably
over 20% by weight, most advantageously about 25% by weight. When
chlorides are used for salt, the appropriate chloride content in
the precipitating solution is in the range 1-30% by weight, prefer-
ably in the range 20-30% by weight. When sulphates are used, the
appropriate sulphate content in the precipitating solution is in
the range 1-30% by weight, most appropriately in the range 20-30%
by weight. The quantity of salt is normally regulated to be such
that the precipitation takes place fast enough, so that the fibre
or film being produced can be spun and stretched.
In addition to an inorganic salt, the precipitating bath may also
contain sodium hydroxide. The sodium hydroxide content must not
become too high because then the precipitating capacity of the bath
is lowered. Consequently, in a continuous process, it is necessary
continuously to remove sodium hydroxlde at the same rate ns lt
enters together with the spinning solution being precipitated. It
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has been observed in the inventton that the precipitating bath may
contaln sodium llydroxi(1e up to 6~ by weight, preferably at most 4%
by weight, be~ore the precipitating capecity is too much impaired.
In the process of the invention, both fibres and films can be pro-
duced from cellulose carbamate, applying the conventional processes
and means oE the viscose method. When producing fibres, the carba-
mate solution is spun through spinnerettes into a precipitating bath,
and when producing films, the solution is pressed through a slit-like
no~zle into the precipitating bath. Since in the procedure of the
invention no acid at all is used, cheaper materials can be used in
the apparatus.
In the procedure of the invention, the carbamate solution being pre-
clpitated contains typically 4-15% by weight cellulose carbamate,
and the appropriate viscosity of the spinning solution is as a rule
in the range 5-50 Ns/m . The appropriate sodium hydroxide concen-
tration is typically in the range 5-12% by weight.
On completion of the precipitation, the after-treatment of the fibres or films, such as washing~ dryin~ and other steps, can be accomplished
using methods and apparatus known in themselves in the
art.
The regeneration of the precipitating bath may be accomplished in a
number of differen~ ways. Appropriate methods have been disclosed,
for instance, in the Finnish patent applications No. 843813 and
843814 of the same applicant, which have been filed concomitantly
with the present patent app?ication.
In the following examples, the procedure of the invention is illus-
trated more in detail. In the examples~cellulose carbamate was used
which had nitrogen content 2.1~ and DP about 300.
Solutions A-D were prepsred from the cellulose carbama~e, their
compositions being presented in Table I below. The solutions were
filtered and de-aired by vacuum treatment. The solutions thus
obtained were used in the spinning experiments.
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Table_I
SolutionCellulose NaOH ~ ZnO Sodium
carbamate content, content, carbonate
content, content,
% % ~ ~
____________________________________________________________
A 7.1 9
B 6.5 9 o.g
C 6.5 9
D 6.8 9 - 2
E 7.4 9
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Exam~le 1
So~ution E was spun at room temperature lnto a precipitating bath
containing 8% sulphuric acid and 20% sodium sulphate. The spinning
was caried out in a laboratory spinnlng apparatus having a nozzle
with 100 holes of 50 um size. The physical characteristics of the
fibre thus obtained were as follows: Titer 1.1 dtex, strength (air-
conditioned) 2.2 cN/dtex, elongation 8~, and wet module 11.4 cN/dtex.
Exam~ 2
Solutions A-D were spun at room temperature in a lsboratory spinning
apparatus having a nozzle with 100 holes of 50 um size. The properties
of the precipitaging solutions and the physical characteristics of
the fibres obtained are presented in Table II.
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The tests show t~lat the process of the invention comparable
results are obtained to those in the case of the product in Example 1,
produced by acid precipitation.
Example 3
A carbamate solution was prepared containg 10% by weight cellulose
carbamate and 9% by weight NaOH. The precipitation took place at
42-44 C in a similar laboratory spinning epparatus as ln Examples
1 and 2. The precipitating bath contained 26% sodium chloride. The
physical characteristcis of the fibre obtained were as follows:
Titre 2.7 dtex, strength 1.5 cN/dtex, elongation 7%, and wet module
8.1 cN/dtex.
E_ ple _
A carbamate solution was prepared containg 7.4% by weight cellulose
carbamate and 10% by weight sodium hydroxide. From the solution,
fibre was spun with a pilot plant spinning apparatus, the spinnerette
20 used therein having 100 holes of 50 um size. The spinning took place
at room temper ~ precipitating bath c-ontaining 25% by
weight sodium . The characteristics of the fibres obtained
were as follows: Titre 2.8 dtex, strength 2.1 cN/dtex, elongation 4%,
and wet module 14 cN/dtex.
Example 5
A carbamate solution was prepared ~containing 6.5% by weight cellulose
carbamate and 9% by weight NAOH and 0.9% by weight ZnO. From the sol-
ution, fibre was spun at room temperature into a water solution, con-
taining 15,4% by weight Na2C03 and 3.1% by weight NaHC03. The charac-
teristlcs of the fibres obtained were as follows: Titre 0.99 dtex,
strength 1.06 cN/dtex, elongation 11% and wet module 12.4 cN/dtex.
Exam ~
A carbamate solution aceording to example 5 was prepared ~nd fibre WaB
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spun from the solution to a water solution containg 12.9% by weight
Na2CO3 and ~ % by weight NaHC03. The characteristics of the fihres
obtained were as follows: Titre 0.99 dt~x, s~lellgth C.86 cN/dtex,
elongation 7% and wet module 12.5 cN/dtex.
Exam ~e 7
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A cellulose carbamate solution was prepared containing 6.6% by weight
cellulose carbamate and 9% by weight NaOH. From the solution flbre
was spun at room temperature to a water solution containing 7.0%
NaHCO3. The characteristics of the fibres of obtained were as follows:
Titre 1,08 dtex, strength 0.74 cN/dtex, elongation 11% and wet module
6.7 cN/dtex.
